Thermodynamic Efficiency of Steady State Operations of MRUs

Abstract

Distillation columns and evaporation equipment are the main energy-consuming components utilized by offshore MRUs, besides several heating and cooling operations. One may ask about what would be the range of expected values of thermodynamic efficiencies for the main MRU operations. A first point to be realized beforehand is that thermodynamic efficiency and exergy efficiency are not in general the same thing, but both have a direct relationship and vary in the same direction. When only energy-consuming processes are considered, as in the case of MRUs, the thermodynamic efficiency expresses the ratio between the minimum consumption of equivalent power to accomplish a given task and the actual consumption of equivalent power for the same task. On the other hand, exergy efficiency is based on the fact that the difference between the outlet usable exergy flow rate of streams and the inlet usable exergy flow rate of streams is related to the minimum consumption (maximum production) of power to accomplish the process task under reversible conditions. The definition of exergy efficiency is similar to the thermodynamic efficiency, but its exergy version can give, in general, different results from the classical thermodynamic efficiency, as exergy counterpart is very dependent on the definition of the environment. This chapter initially approaches the thermodynamic efficiency of a binary idealized distillation column solved by the approximate McCabe–Thiele method, then a real commercial multicomponent distillation is assessed in terms of thermodynamic efficiency, being solved via rigorous distillation package of commercial process simulators with typical specification of products.